In fields such as the aircraft industry, it is very important to be able to effect NDT on various components either after manufacture or after a period of service in order to assess whether any structural faults or metallurgical defects exist. A well established technique has been to use ultrasonic test apparatus. In such apparatus a pulsed ultrasonic signal is transmitted from a transducer into a work piece and the reflected signal recorded in terms of amplitude of received signal as a function of time. The signal is subsequently analysed and interpreted. However, the analysis and interpretation depends upon an accurate knowledge of the frequency characterisation of the transducer system. This varies from transducer to transducer and indeed from installation to installation depending on the cable arrangements, local conditions and other factors even when using transducers of the same fundamental design. Accordingly it has been important to check regularly the frequency characterisation by use of a spectrum analyser and special instrumentation. For example, Aerospace Standard AS1355 of the Society of Automotive Engineers, Inc. of U.S.A. lays down a standard for the use of a spectrum analyser with ultrasonic transducers. Such equipment however, is relatively expensive and such capital investment presents difficulties to operators having a limited number of ultrasonic transducers which only require periodic assessment of their frequency characterisation.
Accordingly, there is a need for an inexpensive and convenient alternative, particularly for users having conventional ultrasonic test equipment. Conventional ultrasonic test equipment includes a cathode ray tube which displays the amplitude of the received signals with respect to time, but does not provide a calibrated time base nor output signal and synchronisation pulses.